The present invention relates to a method and apparatus for generating radio identification information, and more particularly, to encoding and decoding such information using a single inexpensive microprocessor and a modem.
In conventional land mobile radio (LMR) systems, multiple radio transceivers, including for example portable radio units and mobile radio units, initiate and carry on communications over a preselected rf communications channel typically via a base station repeater. Land mobile radios find particular application with police and fire departments, municipalities, and other mobile groups that require immediate communication with other members of their respective group.
FIG. 1 illustrates in block diagram format a typical LMR system. A radio operator transmits a message from sending transceiver 10a to receiving transceiver 10b over a preassigned radio frequency channel. Typically, the radio transceivers 10a and 10b are out of range given the rf transceiving frequency, i.e. on the order of 800 MHz, and therefore require a base station repeater 12 to receive via antenna 14, combiner 16 and receiver 18 the rf transmission from sending transceiver 10a and "repeat" or retransmit the message at a relatively high power via transmitter 20, combiner 16, and antenna 14 for reception by receiving transceiver 10b. The transmit and receive circuits 18 and 20 are controlled by conventional digital and audio processing circuitry 22. A dispatcher at console 24 monitors and participates in radio unit communications. The console 24 includes a display 26, keyboard 28, speaker 30, and microphone 32 via a hardwire link to the base station 12.
In addition to transmitting audio information over a preassigned radio frequency, each radio also transmits at the same time a "channel guard" signal in the form of a low frequency tone pattern or a cyclically repeated digital code pattern. Such a "channel guard" is also referred to as a continuous tone coded squelch system (CTCSS) or a continuous digital coded squelch system (CSCSS). For simplicity and consistency, the term channel guard is used throughout the description and includes CTCSS and CDCSS. The channel guard signal allows an operator to selectively call desired parties and is also used to assign radios programmed on the same communications frequency (e.g. police officers at a local police station) to different groups. Each group of radios is therefore selectively programmed with a particular communications frequency and channel guard frequency tone or digital code pattern.
By using the channel guard option, many users can share a repeater system with specific transceivers being programmed to receive a particular channel guard patterns. The transmitted tones in a tone channel guard system may, for example, range from 67 Hz to 210.7 Hz in 0.1 Hz steps. In a digital channel guard system, there may be over 80 standard digital codes. The frequency of transmitted digital channel guard signals is typically lower than in tone channel guard systems.
Every communication transmitted from a radio unit must be accompanied by a simultaneously transmitted channel guard tone or digital code. Listening radios detect and decode communications on its programmed frequency along with accompanying channel guard signalling, and if the transmitted channel guard signal matches the listening radio's programmed channel guard tone/code, the radio unmutes the radio speaker so the operator may participate in the call. If the channel guard signal does not match the programmed channel guard tone/code, the speaker is muted and the operator does not participate in the received call. If during a communication, a receiving radio (or repeater) no longer detects a channel guard signal, the radio (repeater) terminates the communication. Accordingly, the channel guard signal must be continuously generated and detected in order to establish and conduct a communication.
In addition to generating the channel guard signal when the radio "keys" up a new transmission, each radio also generates it own radio identification number as an initial message every time an operator depresses the push-to-talk (PTT) button to communicate. A conventional messaging format for this radio identification information is the Status Reporting or STAR identifier protocol developed in the 1970's and used in General Electric portable radios. STAR is a digital signalling technique that generates automatic number identification (ANI) information unique to each radio whenever the radio's PTT button is depressed. Thus, preceding each voice transmission (and optionally during and/or at the end of each voice transmission) the digital ANI information is transmitted. Each data burst of ANI information includes a preamble, sync and data bits, the radio's identification number, a message indicating whether the communication is an emergency, and error control codes. For purposes of this description, the term ANI information or ANI signalling is used for simplicity and consistency and includes STAR and GESTAR.TM. signalling formats.
By decoding the ANI information transmitted by a radio, a receiving party such as a central radio dispatcher or another radio is informed of the identity of the radio and whether the communication is an emergency. Thus, the central radio dispatcher immediately knows from the ANI digital signalling if, for example, a police officer sending the message is in trouble (i.e. if an emergency message is included in the ANI information) and the identity of the radio (hence the specific police officer in trouble) before and without the need for the police officer to communicate that information over the voice channel. This is obviously an important feature in many LMR applications.
The simultaneous generation of ANI and channel guard signalling is an important but burdensome data processing task for each radio unit. This is particularly the case in lower-end, conventional radios that employ inexpensive, relatively slow microprocessors. As a result, for a radio to simultaneously generate both ANI and channel guard signals, two such microprocessors are used with one microprocessor dedicated to generating channel guard signals and the other to generating the ANI information. This extra hardware is not a serious problem in the base station repeater (which detects and regenerates these channel guard and ANI signals along with the transmitted voice) where size and cost are less of an issue compared with individual mobile/portable radios. In these radio units, however, it would be desirable reduce the radio size and cost by eliminating one of the microprocessors while still permitting the radio to these two important overhead processing tasks. It would also be advantageous for a radio unit to be able to receive and decode ANI and channel guard information using the same single microprocessors.
The present invention accomplishes these objectives (and others) by taking advantage of a modem built-in to most mobile/portable modem radios. These modem radios are usually manufactured with a uniform set of identical hardware and then subsequently programmed to operate in one or more different operational modes sometimes referred to as "systems." For example, a subscriber may desire a radio that operates in a conventional mode/system as well as a in a trunked mode/system, e.g., single site trunking, multi-site trunking, etc. Since modem radios are outfitted with a standard modem for data communications, (even if they are not actually activated for use in conventional radios), the present invention uses this existing modem under the control of a single inexpensive microprocessor to assist in the transmission and reception of ANI information. In this way, a single, inexpensive microprocessor is able to transmit, receive, and process both ANI and channel guard signals.
A radio communication system services plural radios, each radio including a single microprocessor and a modem and being assigned corresponding radio identification data, a transmission radio frequency, and a corresponding predetermined pattern of tones or digital codes, i.e. a predetermined channel guard. For each radio transmission, the radio microprocessor encodes the radio identification data corresponding to the radio, provides the encoded radio identification data to the radio modem in parallel format, and then generates the predetermined pattern of tones or digital codes corresponding to the radio. The radio modem converts the encoded radio identification data into a serial data stream. The predetermined pattern of tones or digital codes is combined with the serial data stream and simultaneously transmitted over the transmission radio frequency. The radio microprocessor generates the corresponding predetermined pattern of tones or digital codes at the same time the radio modem is converting the encoded identification data into a serial data stream.
When the radio is receiving information, it filters a received signal to separate a received pattern of tones or digital codes from received radio identification data. The filtered radio identification data is converted in the radio modem from a serial data stream into parallel format and stored. The radio microprocessor determines whether the received pattern of tones or digital codes corresponds to the radio's assigned predetermined pattern of tones or digital codes. The radio microprocessor then decodes the stored parallel radio identification data to determine the identification of the transmitting radio. As in the transmission situation, the modem performs the serial to parallel conversion and storage function at the same time that the radio is determining whether the received pattern of tones or digital codes corresponds to the radio's assigned predetermined pattern of tones or digital codes.
The radio communication system in accordance with the present invention finds particular application to LMR mobile radio communication systems which include one or more repeater base stations for receiving and retransmitting radio frequency (rf) communications between plural mobile/portable radios. Each radio includes a memory for storing radio identification data and channel selection information, i.e. ANI and channel guard information. The microprocessor processes the radio identification data retrieved from the memory by spreading each radio identification data bit with a predetermined pattern of chips. Chips are used in the sense that a single ANI bit is represented with a particular binary pattern of chips. The predetermined pattern of chips is based on the data transmission frequency of the modem and the lower frequency of a predetermined (industry standardized) ANI carrier waveform. A binary one is coded using the predetermined pattern of chips, and a binary zero is coded using the complement of the predetermined pattern of chips. In the preferred embodiment of the present invention, the microprocessor further encodes (and decodes) the process radio identification data using differential phase shift keying (DPSK).
The present invention also provides a unique method for generating automatic identification number (ANI) signalling. Each bit of ANI data is represented as a predetermined pattern of chips or as a complement of the predetermined pattern of chips. A modem having a chip transmission frequency greater than the frequency of an ANI carrier waveform is used to repeatedly generate the predetermined pattern of chips and the complement of the predetermined pattern of chips to simulate the ANI carrier waveform .